Network storage arrays can use redundant data copies to ensure data availability. For example, one storage system configuration uses multiple disks to store data. An application host creates new data that is written on a primary mirror disk. A disk controller responds to writes to the primary disk by updating the data changes to a secondary disk automatically. The secondary disk has read-only access from a backup and data mining host system, unless suspended. The mirrored pair has multiple states including an initial creation copy state with full out-of-order copying, a pair state with updated data sent, perhaps out-of-order, a suspended state with consistent and usable but stale data, and a resynchronize state in which data is inconsistent with out-of-order copying. Secondary data is only usable, consistent, and write able during the suspended state.
With existing high-end disk array internal volume copy products, the time duration to transfer all primary volume data to reside on the secondary volume can be very long. At typical internal copy speeds of forty to eighty Megabytes per second, user volumes with a size in the range from hundreds to thousands of gigabytes can last several minutes. During the interim, substantial data loss can occur in the event of a disaster or catastrophe brought on by disturbances as common as a power loss or outage. Users are highly sensitive to the vulnerability inherent in the long copy times that exposes even the primary data to potential loss until the copy completes.
The highly vulnerable copy operation can be a common occurrence for purposes including data warehouse applications, data backup, application testing, and the like so that the loss potential is a frequent worry of users.
Virtual copy techniques exist that simulate or feign completion of the operation before the data has actually transferred. Such techniques utilize frantic out-of-order background copying if the user actually requests the data from the secondary volume. The known techniques have imperfections in that while the secondary volume reader is given the illusion of full data availability, failure of the primary volume prior to completion of a full copy leaves the secondary volume reader with inconsistent and unusable data.